Condensation Inside Your Windows: A GTA Winter Humidity Guide
Condensation on the room side of your windows in winter means your indoor air holds more moisture than the cold glass can tolerate. The fix is to bring humidity down, into Health Canada's 30 to 50% range and toward the lower end on cold snaps, and to keep air moving over the glass with fans, open blinds and ventilation. Moisture between the panes is different: it means a failed sealed unit, and no amount of dehumidifying will clear it.
Condensation on the inside surface is a humidity problem. Between the panes is a failed seal. On the outside surface, it's a sign of good insulation.
Health Canada recommends keeping indoor humidity between 30 and 50%. NRCan adds that it should be lower in winter.
The colder it gets outside, the less humidity a window can handle. Historic NRC data shows double glazing condensing above roughly 32 to 39% RH at −29 °C outside.
A family of four adds about 30 to 50 litres of water to the air each week just by living in the house, before any humidifier.
Bath and kitchen fans, an outside-vented dryer, open blinds and a running HRV fix most cases. Never aim a space heater at cold glass, because the temperature difference can crack it.
Condensation that sticks to one window points to a failed or cold-edged sealed unit, which is fixed by replacing the glass unit in the existing frame. Ice on frames or water rotting the sill needs attention too.
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Condensation on the room side of your windows means your indoor air is holding more moisture than the cold glass can handle. Water condenses whenever a surface temperature falls below the dew point of the room air. Glass is usually the coldest surface in the room, so it gets wet first.
The fix is mostly free. Bring indoor humidity down, especially on the coldest days, and keep air moving across the glass. Health Canada recommends keeping humidity between 30 and 50%, and Natural Resources Canada says to run lower in winter. Here's how to do it in a GTA home, and how to tell when the window, not the air, is the problem.
Inside, outside or between the panes?
Where the water sits tells you what's wrong:
| Where you see it | What it means | What to do |
|---|---|---|
| On the room-side surface | Indoor humidity is too high for the glass temperature | Lower humidity, improve air flow (this guide) |
| Between the panes (won't wipe off) | The sealed unit's seal has failed | Replace the sealed unit. See how to tell if your seal is broken. |
| On the outside surface | Dew; the outer pane is well insulated from the heat indoors | Nothing. It's a sign of efficient glass. |
Why GTA windows sweat in winter
Warm air holds more moisture than cold air. When room air touches glass colder than its dew point, the extra moisture comes out as water or frost. The colder it is outside, the colder your inner pane gets, and the less humidity it takes to fog it.
GTA winters are cold enough for that to matter. January averages −6.3 °C at Pearson and −4.2 °C downtown, and the city still sees about 2 to 3 days a year below −20 °C. North of the city it's colder again. Newmarket needs about 4,260 heating degree-days a year against 3,520 at Toronto City Hall, so the same humidity that's fine downtown can fog windows in York Region.
How much humidity can your windows handle?
The classic Canadian data comes from National Research Council building digests published in 1960. The numbers are old and modern low-E windows do better, but the pattern still holds. With the room at 21 °C, this is the highest indoor relative humidity (RH) the glass tolerated before condensing:
| Outdoor temperature | Single glazing | Double glazing | Triple glazing |
|---|---|---|---|
| −18 °C (0 °F) | 12–27% RH | 41–49% RH | 57–61% RH |
| −29 °C (−20 °F) | 6–17% RH | 32–39% RH | 49–53% RH |
Two takeaways. First, a humidity level that's comfortable in November can fog your windows in a January cold snap. Second, better glazing buys you headroom. At −10 °C outside, the inner glass of a double-pane window sits at 11 to 14 °C, against 16 to 18 °C for triple pane.
Published targets differ slightly. Health Canada and Natural Resources Canada both put indoor humidity at 30–50%, with the caveat that it should run lower in winter and higher in summer. In colder, drier regions 25–40% in winter is the usual advice, and a common winter rule of thumb is 30–35%.
A practical rule for GTA homes is to stay inside Health Canada's range and drift toward its low end as the temperature drops. Condensation on your windows is itself a good indication of your humidity level.
Where the moisture comes from
Living in a house puts a lot of water into the air. Natural Resources Canada estimates a family of four produces about 50 litres a week, 38 of it just from breathing and skin evaporation. Other estimates put it at 30 to 40 litres a week, with showers, cooking and bathing adding 15 to 20 litres. Add a humidifier, indoor laundry drying, aquariums or lots of plants, and it climbs fast.
New windows can make it worse. Many homeowners find condensation increases after replacing windows, because the old leaky ones actually helped to reduce humidity.
How to stop condensation on your windows
- Measure first. Put an inexpensive hygrometer in the main living area and one in a bedroom. Measure rather than guess.
- Turn the humidifier down or off when windows start to fog, and check any humidifier on the furnace.
- Run the bathroom fan during every shower and for a while after. Use the range hood whenever you cook, especially when boiling.
- Vent moisture-producing appliances outdoors, especially the clothes dryer. Drying laundry on racks indoors adds moisture too.
- Open blinds and curtains so room air can circulate over the glass. Closed coverings trap cold, damp air against it.
- Run your HRV if you have one. A heat-recovery ventilator swaps stale, humid indoor air for drier outdoor air while recovering much of the heat.
- Fix drafts around the sash. Worn weatherstripping chills the frame and glass edges. Air leakage also accounts for 30 to 40% of a home's heating and cooling loss.
- Don't aim heat directly at the glass. A space heater or vent blowing on a window, or a half-closed blind, creates sharp temperature differences across the pane that can cause thermal stress cracks. Keep blinds fully open or fully closed. Our guide to why windows crack explains the risk.
For a wider cold-weather list, see our GTA winter checklist for windows and doors.
When condensation means the window needs work
Humidity is the usual cause, but sometimes the window is part of the problem:
- Moisture between the panes. The sealed unit has failed. It won't clear with any change in humidity. The fix is a new sealed unit, not a new window; see foggy windows: replace the glass, keep the frame.
- One window is always worse than the others. A unit whose seal has failed can lose heat up to 32% faster, so its room-side glass runs colder. Units with old aluminum spacers also run cold at the edges. Warm-edge spacers reduce edge condensation compared with conventional metal ones.
- Ice or water on the frame, not just the glass. Look for drafts, worn weatherstripping, or an old metal frame without a thermal break.
- Water pooling on sills, peeling paint or soft wood. Deal with it now, before it damages the sill, drywall or wall cavity.
Renting? Toronto's property standards by-law requires window openings to be maintained in a weather-tight condition to prevent drafts or leakage. Tell your landlord in writing about failed seals or drafts. Condensation caused by high indoor humidity is usually something the occupant controls. This is general information, not legal advice.
What we'd do
If you've brought humidity into range and a window still runs wet, we'd look at the window itself. We check whether the sealed unit has failed, whether the spacer is an old metal type, and whether the sash seals tightly. Where the glass is the problem, a new sealed unit with low-E glass, argon and a warm-edge spacer, fitted to your existing frame, warms the room-side surface. Our low-E, argon and triple-pane guide explains the options. If years of water on the sill have left the frame itself rotted or soft, that's a window-replacement job, not a glass repair, and we'll tell you if that's where you are.
Next step
Room-side condensation is usually a humidity fix you can do yourself. Fog between the panes isn't: that needs a new sealed unit, not a new window. See our foggy window repair service, where we replace the sealed glass unit and keep your frame, or contact us with a photo and we'll tell you which one you're dealing with.